Heterojunction iii-v solar cell performance

ABSTRACT

An In x Ga 1-x As interlayer is provided between a III-V base and an intrinsic amorphous semiconductor layer of a heterojunction III-V solar cell structure. Improved surface passivation and open circuit voltage may be obtained through the incorporation of the interlayer within the structure.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/179,731 filed Jul. 11, 2011, the complete disclosure of which isexpressly incorporated herein by reference in its entirety for allpurposes.

FIELD OF THE INVENTION

The present invention relates to the physical sciences, and, moreparticularly, to the field of heterojunction solar cells.

BACKGROUND OF THE INVENTION

Direct gap III-V semiconductor materials have strong absorptionproperties and have been found to be suitable for high efficiency solarcell applications, particularly portable applications. Small thicknessesof these materials, in the order of a few microns, are sufficient forabsorbing a large portion of the solar spectrum. Conventional techniquesfor making high efficiency III-V solar cells can involve the epitaxialgrowth of relatively complicated structures using MOCVD (metalorganicchemical vapor deposition) and MBE (molecular beam epitaxy) systems.Despite the high efficiency of epitaxially grown III-V solar cells, thehigh cost of epitaxy presents challenges to the production ofcost-competitive III-V solar cells for terrestrial applications.

GaAs has a relatively wide energy gap (1.4 eV) and in turn offers a highopen circuit voltage. Such properties make it suitable for singlejunction solar cells. GaAs further exhibits high absorption of the solarspectrum that can be converted to a high short circuit current.

SUMMARY OF THE INVENTION

Principles of the invention provide a heterojunction III-V solar cellstructure including an interlayer that is effective in reducing thecontribution of the surface recombination in the dark current within thestructure and improving the open circuit voltage. Techniques forfabricating such a structure are further provided. In accordance with afirst aspect, a method is provided that includes obtaining a base layercomprising one of gallium arsenide and In_(y)Ga_(1-y)As where y<x,having a first doping type, forming an epitaxial In_(x)Ga_(1-x)As layeron the base layer where 0.01<x<1 having the first doping type, formingan intrinsic hydrogenated amorphous silicon semiconductor layer on theepitaxial In_(x)Ga_(1-x)As layer, and forming a doped hydrogenatedamorphous silicon layer on the intrinsic semiconductor layer. The methodmay further include forming a transparent conductive layer on theamorphous hydrogenated silicon layer. The intrinsic semiconductor layercan contain germanium and carbon atoms and can be comprised of aplurality of intrinsic semiconductor layers, one or more of which mayinclude germanium and carbon atoms. The doping type of the hydrogenatedamorphous silicon layer formed on the intrinsic semiconductor layer maybe the opposite type of the base doping type if used to form an emitterand can be the same type as the base if used to form a back surfacefield (BSF).

In another aspect, an exemplary heterojunction III-V solar cellstructure includes a base layer comprising gallium arsenide, anintrinsic semiconductor layer such as intrinsic hydrogenated amorphoussilicon, an epitaxial In_(x)Ga_(1-x)As interlayer between andinterfacing with the base layer and the intrinsic semiconductor layer,wherein 0.01<x<1.0, a doped hydrogenated amorphous silicon layer on theintrinsic hydrogenated amorphous silicon layer, and a transparentconductive layer above the doped hydrogenated amorphous silicon layer.The base layer and the In_(x)Ga_(1-x)As interlayer have a first dopingtype while the doped hydrogenated amorphous silicon layer has a seconddoping type opposite from the first doping type if used to form theemitter. However, if it is used to form a back-surface field, the dopedhydrogenated amorphous silicon layer has the first doping type as well.

In a still further aspect, a heterojunction III-V solar cell structurethat includes a base layer comprising one of GaAs and In_(y)Ga_(1-y)Aswhere y<x, an epitaxial In_(x)Ga_(1-x)As interlayer between andinterfacing with the base layer and the intrinsic hydrogenated amorphoussilicon layer, wherein 0.01<x<1.0. The base layer and theIn_(x)Ga_(1-x)As interlayer have the same doping type. A dopedhydrogenated amorphous silicon layer is on the intrinsic hydrogenatedamorphous silicon layer and a transparent conductive layer is on thedoped hydrogenated amorphous silicon layer.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

One or more embodiments of the invention or elements thereof can beimplemented in the form of a computer program product including atangible computer readable recordable storage medium with computerusable program code for performing the method steps indicated.Furthermore, one or more embodiments of the invention or elementsthereof can be implemented in the form of a system (or apparatus)including a memory, and at least one processor that is coupled to thememory and operative to perform exemplary method steps. Yet further, inanother aspect, one or more embodiments of the invention or elementsthereof can be implemented in the form of means for carrying out one ormore of the method steps described herein; the means can include (i)hardware module(s), (ii) software module(s), or (iii) a combination ofhardware and software modules; any of (i)-(iii) implement the specifictechniques set forth herein, and the software modules are stored in atangible computer-readable recordable storage medium (or multiple suchmedia).

Techniques of the present invention can provide substantial beneficialtechnical effects. For example, one or more embodiments may provide oneor more of the following advantages:

-   -   Reducing the contribution of the dark current component due to        surface recombination at the heterointerface;    -   Improving open circuit voltage (V_(oc));    -   Reducing surface recombination velocity (SRV) at the        heterointerface;    -   Improving the short circuit density and fill factor.

These and other features and advantages of the present invention willbecome apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a single and double heterojunction solar cellstructures, respectively;

FIG. 2 shows a portion of the structures shown in FIGS. 1A and 1B;

FIG. 3 shows a solar cell structure in accordance with an embodiment ofthe invention;

FIGS. 4A and 4B are energy band diagrams comparing two solar cellstructures, one of which includes an InGaAs layer;

FIG. 5 is a chart including a comparison of surface recombinationvelocity for Si, GaAs and InGaAs, and

FIG. 6 is a flow chart showing manufacturing steps for a heterojunctionIII-V solar cell structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Heterojunction III-V solar cell structures based on a-Si:H/III-Vheterostructures offer a path for low-cost, high efficiency PV(photovoltaic) technology when implemented in conjunction with a layertransfer technique. The use of a-Si:H as an intrinsic layer (i-a:Si:H)can significantly improve the surface passivation of GaAs. US Pub. No.2010/0307572 entitled “Heterojunction III-V Photovoltaic CellFabrication”, the disclosure of which is incorporated by referenceherein, discloses techniques for forming single and doubleheterojunction III-V PV cells.

FIGS. 1A and 1B show single and double heterojunction solar cellstructures 10, 30, respectively, that can be produced using techniquesdisclosed in US Pub. No. 2010/0307572. Referring to FIG. 1A, thestructure 10 includes a base layer 12 comprising a III-V substrate thatmay be n-type or p-type. An optional BSF (back surface field) layer 14is formed on the base layer. A doped amorphous hydrogenated siliconlayer 18 is formed on an intrinsic amorphous silicon layer 16 on thebase layer 12. The doped amorphous hydrogenated silicon layer 18 mayhave a doping type opposite from the doping type of the base layer. Atransparent layer 20 such as a transparent conducting oxide (TCO) isformed above and preferably on the amorphous silicon layer 18. Frontmetal contacts including fingers and bus bars (not shown) may be formedon the transparent layer. Although, the i-a:Si:H layer 16 improvessurface passivation of a GaAS base layer, the relatively high interfacetrap density at the a-Si/GaAs heterointerface leads to a significantlyhigh dark current, degrading the open circuit voltage.

The double-heterojunction structure 30 shown in FIG. 1B includes twobase/i-a:Si:H interfaces and two doped, amorphous hydrogenated siliconlayers 18, 32. If the doped a-Si:H layer is used to form an emitter, itsdoping type is opposite to that (n-type or p-type) of the base layer. Ifused to form a BSF, its doping type would be the same as the base layer.As discussed above with respect to the single-heterojunction structure10, high interface state density limits the open circuit voltage. Inthese particular examples, such voltage (V_(oc)) may be in the range of500-600 mV.

FIG. 2 shows a structure 40 comprising a portion of the structures 10,30 shown in FIGS. 1A and 1B and is provided for purposes of comparisonto the embodiment of the invention shown in FIG. 3. FIG. 3 shows acorresponding portion of a solar cell structure 50 according to anexemplary embodiment of the invention, which may comprise part of asingle or double-heterojunction solar cell structure. The structure 50of FIG. 3 includes an epitaxial In_(x)Ga_(1-x)As interlayer 52 asdescribed further below. This interlayer 52 is formed on the base layer.Such an interlayer 52 helps reduce the contribution of a dark currentcomponent due to surface recombination at the heterointerface andimproves V_(oc).

The structure 50 of the exemplary embodiment of the invention shown inFIG. 3 includes an In_(x)Ga_(1-x)As interlayer 52 that facilitates solarcell efficiency. The In_(x)Ga_(1-x)As interlayer can be grownepitaxially using conventional chemical vapor deposition (CVD) methodssuch as metal organic CVD (MOCVD) or molecular beam epitaxy (MBE). Therange of x is 0.01<x<1, preferably 0.05<x<0.8. The interlayer 52preferably has a thickness between 5-50 nm. In the exemplary embodimentof FIG. 3, it is 5-20 nm. The doping range for the In_(x)Ga_(1-x)Asinterlayer is 1e14-1e18/cm³ and can be different from that of the baselayer 12.

The base layer 12 provided in the exemplary embodiment of FIG. 3 iscomprised of GaAs or In_(y)Ga_(1-y)As, with the In_(x)Ga_(1-x)Asinterlayer 52, where y<x. The base doping level is from1e14-1e18/cm^(3.) The doping type (n-type or p-type) of the dopedamorphous silicon layer 18 is opposite to that of the base andIn_(x)Ga_(1-x)As layers if it is used to form an emitter and is the sameif it is used to form the back surface field. The base layer is between0.2-30 μm in thickness in this exemplary embodiment. The n-type dopantconcentration in certain layer(s) of the structure 50 ranges from 10¹⁶atoms/cm³ to 10²¹ atoms/cm³, with the range of 10¹⁸-10²⁰ atoms/cm³ beinga typical range. The doping efficiency (the ratio of activated dopantatoms to the total dopant atoms) typically ranges from 0.1%-20%,although higher and lower doping efficiencies are possible. The dopingefficiency is generally decreased by increasing the dopant atomconcentration. The p-type dopant concentration likewise ranges from10¹⁶-10²¹ atoms/cm3 with the range of 10¹⁸-10²⁰ atoms/cm³ being typical.

The transparent conductive layer 20 includes a conductive material thatis transparent in the range of electromagnetic radiation at whichphotogeneration of electrons and holes occur within the solar cellstructure. The transparent conductive layer 20 in the exemplaryembodiment of FIG. 3 may comprise TCO's such as indium tin oxide (ITO),tin oxide (SnO), fluorine-doped tin oxide (SnO₂:F) or aluminum-dopedzinc oxide (ZnO:Al). Transparent conducting films such as carbonnanotube-based films and graphene-based films may alternatively beemployed to form this transparent conductive layer 20. These examplesare to be considered exemplary as opposed to limiting. The thickness ofthe transparent conductive layer may vary depending on the type oftransparent conductive material employed as well as the technique usedin forming this layer. Typically, and in one exemplary embodiment, thethickness of the transparent conductive layer 20 is between 20-500 nm.Other thicknesses, including those less than 20 nm and/or greater than500 nm, can alternatively be employed. The preferred thickness of thetransparent conductive layer for minimizing reflection from the surfaceof Si is in the range of 70-110 nm for a TCO. Nanotube andgraphene-based films may in the range of 2-50 nm. The transparentconductive layer is typically formed using a deposition process such assputtering or CVD. Examples of CVD processes usable for a number oftypes of such layers include APCVD, LPCVD, PECVD, MOCVD and combinationsthereof RF and DC magnetron sputtering are among other techniques forforming the transparent conductive layer 20.

The intrinsic amorphous hydrogenated silicon semiconductor layer 16formed on the In_(x)Ga_(1-x)As interlayer lacks a crystal structure withlong range order. This layer can be referred to as an intrinsicsemiconductor or undoped or i-type semiconductor, that is asubstantially pure semiconductor without any significant electricaldopant impurity present. The number of charge carriers in the intrinsicsemiconductor is determined by the properties of the material itselfrather than the amount of impurities, i.e. dopants. The intrinsichydrogenated amorphous semiconductor layer 16 provided in the embodimentof FIG. 3 is formed using any suitable physical or chemical vapordeposition process including appropriate source materials. In oneembodiment, the intrinsic hydrogenated semiconductor layer is depositedin a process chamber containing a semiconductor precursor source gas anda carrier gas including hydrogen. Hydrogen atoms within the carrier gasare incorporated into the deposited material to form the intrinsichydrogenated semiconductor containing material of the intrinsicsemiconductor layer. The intrinsic semiconductor layer 16 may containgermanium and carbon atoms. The germanium and carbon distribution can begraded or constant throughout the layer. The intrinsic semiconductorlayer 16 can be a multi-layer structure comprising a combination oflayers, one or more of which contain graded or ungraded germanium andcarbon and one or more of which do not contain these elements.

The hydrogenated amorphous silicon layer 18 may be formed from precursorgases such as SiH₄.SiF₄, or H₂SiCl₂ (DCS). The layer may be doped “insitu” by adding a dopant gas containing dopant atoms in the gas mixture.The dopant atoms are incorporated into the deposited material to form ahydrogenated doped semiconductor. Examples of dopant gases containingp-type dopant atoms are B₂H₆ and B(CH₃)₃ (TMB). Examples of an n-typedopant gas include AsH₃ and PH₃.

FIGS. 4A and 4B provide theoretical energy band (i.e. conduction band,valence band and Fermi energy) diagrams comparing solar cell structureswith and without the epitaxial In_(x)Ga_(1-x)As interlayer. The improvedsurface passivation using the epitaxial In_(x)Ga_(1-x)As interlayer isbelieved to be due to lower density of active interface traps at thei-a:Si:H/In_(x)Ga_(1-x)As interface compared to that of an i-a:Si:H/GaAsinterface and additional field-effect passivation at theIn_(x)Ga_(1-x)As/GaAs interface due to the band discontinuity.

FIG. 5 provides a comparison of SRV (surface recombination velocity) forIn_(x)Ga_(1-x)As (specifically In_(0.53)Ga_(0.47)As) vs. GaAs. Thereduction of the SRV at the heterointerface using the epitaxialIn_(x)Ga_(1-x)As interlayer is believed attributable to the moreforgiving nature of In_(x)Ga_(1-x)As, which is easier to passivate ascompared to GaAs and the field effect induced passivation due to theband discontinuity at the In_(x)Ga_(1-x)As/GaAs interface. Prior to thedeposition of the i-a-Si:H layer 16, the native oxide should be removedfrom the surface of the III-V layer. III-V materials are prone to rapidoxidation in the presence of oxygen. To prevent the formation of thenative oxide, the oxide desorption and the subsequent processing step,i.e. deposition of i-a-Si:H, should be done in situ using an ultra highvacuum (UHV) system. Solar cell processing costs may, however,necessitate ex situ desorption of the native oxide. As a result, theformation of some native oxide during sample transfer in air fori-a-Si:H deposition can be inevitable. If some native oxide is formedduring the sample transfer, the SRV between the III-V material and itsnative oxide plays a role in the total dark current of the structure.

The solar cell structure 50 described herein can constitute part of asingle or double heterojunction cell structure. In the case of a doubleheterojunction cell structure, an In_(x)Ga_(1-x)As interlayer would beprovided on both sides of the base layer.

FIG. 6 is a flow chart showing the steps of forming the solar cellstructure 50. As discussed above, the base layer 12 can be providedusing any layer transfer technique from a GaAs substrate in step 102.The In_(x)Ga_(1-x)As layer is deposited epitaxially on the base layervia CVD or MBE in step 104. The intrinsic layer is formed by a physicalor chemical vapor deposition process in step 106. The doped hydrogenatedamorphous silicon layer 18 may be formed from precursor gases in a CVDprocess in step 108. The transparent conductive layer is formed using adeposition process such as sputtering or CVD in step 110. One or more ofthe steps 104, 106, 108 and 110 may precede or follow formation of thebase layer. The method according to the invention accordinglyencompasses forming the epitaxial In_(x)Ga_(1-x)As layer on the baselayer whether or not the base layer is still part of the substrate.

Given the discussion thus far and with reference to FIG. 6, it will beappreciated that, in general terms, an exemplary method, according to anaspect of the invention, includes the step of obtaining a base layercomprising one of gallium arsenide and In_(y)Ga_(1-y)As where y<x,having a first doping type, forming an epitaxial In_(x)Ga_(1-y)As layeron the base layer where 0.01<x<1 having the first doping type, formingan intrinsic semiconductor layer on the In_(x)Ga_(1-x)As layer, andforming a doped hydrogenated amorphous silicon layer on the intrinsicsemiconductor layer. A transparent conductive layer may be formed on theamorphous hydrogenated silicon layer in a further step. The dopedhydrogenated amorphous silicon layer has a doping type opposite to thatof the base layer if used as an emitter and has a doping type similar tothat of the base layer if used as a back-surface field.

A heterojunction III-V solar cell structure is further provided inaccordance with another aspect of the invention. The structure comprisesa base layer comprising one of GaAs and In_(y)Ga_(1-y)As where y<x, anintrinsic semiconductor layer, and, an epitaxial In_(x)Ga_(1-x)Asinterlayer wherein 0.01<x<1.0 located between and interfacing with thebase layer and the intrinsic semiconductor layer. The base layer and theIn_(x)Ga_(1-x)As interlayer have the same doping type. A dopedhydrogenated amorphous silicon layer is on the intrinsic semiconductorlayer and a transparent conductive layer is on the doped hydrogenatedamorphous silicon layer. FIG. 3 shows an exemplary embodiment of such astructure wherein the intrinsic semiconductor layer is shown asi-a-Si:H, the transparent conductive layer 20 is shown as a TCO, andspecific thickness ranges of certain layers are provided. As discussedabove, the thickness range In_(x)Ga_(1-x)As interlayer is 5-50 nm, butis deposited in a narrower range in the embodiment of FIG. 3.

In accordance with a further aspect of the invention, a heterojunctionIII-V solar cell structure comprises a base layer comprising galliumarsenide, an intrinsic semiconductor layer, and an epitaxialIn_(x)Ga_(1-x)As interlayer between and interfacing with the base layerand the intrinsic semiconductor layer, wherein 0.01<x<1.0. A doped,hydrogenated amorphous silicon layer is on the intrinsic semiconductorlayer and a transparent conductive layer is above the hydrogenatedamorphous silicon layer. Both the intrinsic semiconductor layer 16 andthe doped layer 18 may contain germanium and carbon atoms (e.g.a-Si_(1-x′-y′)Ge_(x′)C_(y′):H where x′ is between 0 and 1 and preferablybetween 0 and 0.5 and y′ is between 0 and 0.6 and preferably between 0and 0.3.). As discussed above, the intrinsic semiconductor layer can bea multi-layer structure. The doping type of the base layer and theIn_(x)Ga_(1-x)As interlayer is the same. In other words, they have a“first” doping type that can be “n” or “p”. The hydrogenated amorphoussilicon layer has a second doping type opposite from the first dopingtype in this embodiment.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. Words such as “above” or “below”refer to relative positions as opposed to altitude unless specifiedotherwise.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1-10. (canceled)
 11. A heterojunction III-V solar cell structure,comprising: a base layer comprising one of GaAs and In_(y)Ga_(1-y)As; anintrinsic semiconductor layer; an epitaxial In_(x)Ga_(1-x)As interlayerbetween and interfacing with the base layer and the intrinsicsemiconductor layer, wherein 0.01<x<1.0 and y<x, the base layer and theIn_(x)Ga_(1-x)As interlayer having the same doping type; a dopedhydrogenated amorphous silicon layer on the intrinsic semiconductorlayer, the doped hydrogenated amorphous silicon layer having a dopingtype with respect to the base layer to function as one of an emitter orback surface field operatively associated with the base layer, and atransparent conductive layer on the doped hydrogenated amorphous siliconlayer.
 12. The heterojunction III-V solar cell structure of claim 11wherein the base layer comprises GaAs.
 13. The heterojunction III-Vsolar cell structure of claim 11 wherein the base layer comprisesIn_(y)Ga_(1-y)As.
 14. The heterojunction III-V solar cell structure ofclaim 11, wherein 0.05<x<0.8.
 15. The heterojunction III-V solar cellstructure of claim 14, wherein the intrinsic semiconductor layercomprises hydrogenated amorphous silicon.
 16. The heterojunction III-Vsolar cell structure of claim 15, wherein the hydrogenated amorphoussilicon in the intrinsic semiconductor layer comprises germanium andcarbon atoms.
 17. The heterojunction III-V solar cell structure of claim15, wherein the In_(x)Ga_(1-x)As interlayer has a thickness between 5-50nm.
 18. The heterojunction III-V solar cell structure of claim 11,wherein the In_(x)Ga_(1-x)As interlayer has a thickness between 5-50 nm.19. The heterojunction III-V solar cell structure of claim 11, whereinthe hydrogenated amorphous silicon layer has a doping type opposite tothe doping type of the base layer and is functional as an emitteroperatively associated with the base layer.
 20. The heterojunction III-Vsolar cell structure of claim 11, wherein 0.05<x<0.8 and the intrinsicsemiconductor layer comprises a plurality of hydrogenated amorphoussilicon layers.
 21. The heterojunction III-V solar cell structure ofclaim 11, wherein the base layer comprises GaAs and the intrinsicsemiconductor layer comprises hydrogenated amorphous silicon.
 22. Aheterojunction III-V solar cell structure, comprising: a base layercomprising gallium arsenide; an intrinsic semiconductor layer; anepitaxial In_(x)Ga_(1-x)As interlayer between and interfacing with thebase layer and the intrinsic semiconductor layer, wherein 0.01<x<1.0; adoped hydrogenated amorphous silicon layer on the intrinsicsemiconductor layer, and a transparent conductive layer above thehydrogenated amorphous silicon layer, the base layer and theIn_(x)Ga_(1-x)As interlayer having a first doping type, the dopedhydrogenated amorphous silicon layer having a second doping typeopposite from the first doping type and being operatively associatedwith the base layer as an emitter.
 23. The heterojunction III-V solarcell structure of claim 22, wherein 0.05<x<0.8.
 24. The heterojunctionIII-V solar cell structure of claim 22 wherein the intrinsicsemiconductor layer comprises hydrogenated amorphous silicon.
 25. Theheterojunction III-V solar cell structure of claim 24 where in theIn_(x)Ga_(1-x)As interlayer has a thickness between 5-50 nm.